1. Field of the Invention
The present invention relates to a small DC motor which is reduced in volume and is made to be easily mounted.
2. Background Art
In DC motors which are used in laser printers, inkjet printers and other electrical equipment, there exists a demand that motors be reduced in size while the properties thereof are maintained.
Due to this, the external shape of a motor is made smaller in such a state that a magnetic constituent portion (outside diameter and lamination thickness of an armature core) of a rotating element which determines the properties of the motor is maintained by changing the shape of a magnetic constituent portion (magnet, motor frame) of a stationary element.
For a DC motor to rotate, since a ratio of the number of magnetized magnetic poles (N: a positive arbitrary number) to the number of slots (M: a positive arbitrary number) basically becomes 2N:3M, the number of magnetized magnetic poles of a magnet is 2 poles, 4 poles, 6 poles, 8 poles, . . . . Generally, DC motors are formed into oval and cylindrical shapes, and in the event of a motor of an oval shape, a magnet is magnetized to 2 poles, whereas in the event of a motor of a cylindrical shape, a magnet is magnetized to 2 to 2N poles. For the cylindrical motor to be reduced in size, the grade (maximum energy product: (BH)max) is increased, the thickness of the magnet is reduced and the outside diameter of a core is reduced while the properties of the motor are maintained. However, in the event that the thickness of the magnet is changed, it is extremely difficult, in most cases, to maintain the motor properties only by upgrading the grade of the magnet.
In addition, in the motors which are required to be reduced in size, motor frames are formed as small as possible due to the necessity of narrowing the accommodation space. Due to this, in many cases, motor frames are formed into cylindrical and oval shapes so as to match the shapes of rotors, and also in motor frames of a type in which field magnets are mounted thereon, the cylindrical and oval shapes are similarly used.
A related art is disclosed in JP-A-07-059322 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) below which attempts to realize the reduction in the external size of a motor and easy mounting thereof while maintaining the properties of a motor. By forming a motor frame into a quadrangular shape having the same number of corner portions as the number of magnetized magnetic poles of a magnet, the reduction in size of a motor is enabled without changing the external shape of an armature core and furthermore without reducing the thickness of central portions of the poles of the magnet so as to maintain the properties of the motor.
FIG. 8 is a cross sectional view of a small DC motor described in JP-A-7-059322.
A small DC motor 100 in FIG. 8 comprises a quadrangular cylindrical motor frame 101 having a substantially square cross section, a magnet 102 accommodated inside the motor frame 101 and having a circular cylindrical space therein which has four magnetic poles which are magnetized alternately to N pole and S pole in such a manner that centers of the magnetic poles come to lie in corner portions of the quadrangular cylindrical motor frame, respectively, a rotor 103 which is accommodated in the circular cylindrical space in the magnet 102 via a radial gap, and a feeding terminal (not shown) which is drawn out of one side of the quadrangular cylindrical motor frame 101 so that the motor is placed horizontally on an printed circuit board (not shown) for use.
In the small DC motor 100 disclosed in JP-A-07-059322 which has the four-magnetic-pole field magnet 102 which is magnetized to have the four magnetic poles which are magnetized alternately to N pole and S pole, the motor frame which holds the field magnet 102 is formed so as to have the substantially square cross section, and the field magnet 102 is magnetized so that the centers of the magnetic poles come to lie in the corners of the square. A magnet used as the field magnet 102 is made up of a plastic magnet and is molded together with the quadrangular cylindrical motor frame 101.
According to this configuration, since the motor takes the angular or quadrangular prism shape, the motor can be fixed to a printed circuit board or the like by being bonded thereto by means of a adhesive double coated tape. Since the thickness of the magnet 102 at the portions thereof which lie in the corners of the quadrangular shape is increased, an actual permeance coefficient is increased.
In the small DC motor described in JP-A-07-059322, however, since the magnet is filled even to the four corners of the motor frame, the thickness of the magnet is such as to be more than what is needed by an actual driving property. Namely, when viewed in a radial direction from the center of rotation, the thickness of the magnet becomes thickest at the portions of the magnet which correspond to the four corner portions of the motor frame and exceeds a magnet thickness where a sinusoidal property needed by a magnetic property (a magnetic flux density property) is formed. Due to this, the magnet portions which correspond to the four corner portions of the motor frame can be cut by such an extent that there is caused no problem with the magnetic property.
In addition, since an advantage is said to be provided that when fixing a motor frame, the motor frame can easily be fixed in the event that the motor frame has plane sides, the advantage is understood to be provided overall in the event that part of each of the original plane sides of the motor frame remains intact. The aforesaid related art example has problems with these points.